The experimental evaluation of therapeutic practices by means of randomized clinical trials (RCTs) has been widely proposed as a means of placing medical practice and health planning on a more rational, scientific basis. Because of their formal advantages over non-randomized studies, RCTs are expected to substantially reduce controversy over the merits of competing therapies. Yet in recent years, a number of long-term RCTs of therapies for chronic disease (diabetes, coronary heart disease, stroke) have been subject to extended controversy. Most discussions dealing with these contested trials have focused on the technical shortcomings of experimental design or debated issues in statistical analysis and interpretation. The hypothesis of the proposed study, derived from recent work in the history and sociology of science regarding the use and evaluation of evidence in scientific controversies, is that these technical debates are grounded in specific ideological concepts of medical practice and research, which in turn derive from the disciplinary backgrounds of diverse groups within the bio-medical community. Groups with distinctive professional backgrounds are expected to hold different concepts of medical science, medical research and the objectives of medical practice. We hypothesize that these conceptions in turn strongly influence the response to individual empirical studies. The proposed study offers a detailed analysis of the cognitive and social bases for two areas of therapeutic controversy: the extended debate over the University Group Diabetes Program RCT and disputes about a series of RCTs evaluating various treatments for the secondary prevention of myocardial infarction. The latter group of studies, while far less controversial, have been subject to many of the same criticisms which plagued the UGDP. Through extensive interviewing of participants in these controversies, supplemented by analysis of published writings, we expect to identify the criteria employed in interpreting the results of specific trials, and to relate these views to prior held beliefs both about the role of statistical inference in medical research, the relation between evidence from randomized and non-randomized studies, and the underlying pathophysiology of the disease in question. Finally, we will attempt to account for these cognitive differences by examining the professional background and institutional circumstances for participants in these controversies.